1. Field of the Invention
The present invention relates to a technique for driving a cold cathode fluorescent tube employing a piezoelectric transformer for use in backlight devices of liquid crystal panels such as personal computers, liquid crystal monitors, and liquid crystal televisions. In particular, the present invention relates to a light emission device for controlling the driving of a plurality of cold cathode fluorescent tubes with a plurality of piezoelectric transformers.
2. Description of the Related Art
Piezoelectric transformers have the characteristics that when a load is infinite, a very high voltage step-up ratio can be obtained, and when a load becomes smaller, the voltage step-up ratio is decreased. The piezoelectric transformers have advantages such as having higher power density than that of electromagnetic transformers so that compactness can be achieved, being non-combustible, and not generating noise due to electromagnetic induction. From the above-described characteristics, the piezoelectric transformers recently have been used as a power source for a cold cathode fluorescent tube.
FIG. 21 shows the configuration of a Rosen type piezoelectric transformer, which is a typical structure of a conventional piezoelectric transformer. This piezoelectric transformer includes a low impedance portion 1, a high impedance portion 2, input electrodes 3U and 3D, an output electrode 4, and piezoelectric elements 5 and 7. The polarization direction of the piezoelectric element 5 in the low impedance portion 1 is denoted by PD, and the polarization direction of the piezoelectric element 7 in the high impedance portion 2 is denoted by PL.
The low impedance portion 1 of the piezoelectric transformer is an input portion when the transformer is used to step up a voltage. In the low impedance portion 1, polarization is provided in the thickness direction as shown in the polarization direction PD, and the electrodes 3U and 3D are provided on the principal surface and the back thereof, respectively, in the thickness direction. The high impedance portion 2 is an output portion when the transformer is used to step up a voltage. In the high impedance portion 2, polarization is provided in the longitudinal direction as shown in the polarization direction PL, and the electrode 4 is provided in the end face in the longitudinal direction. When a predetermined alternating voltage is applied between the electrodes 3U and 3D, the thus configured piezoelectric transformer excites vibration that expands and contracts in the longitudinal direction and converts this vibration to a voltage generated between the electrodes 3U and 4 by the piezoelectric effect. The voltage is stepped up and down by the impedance conversion with the low impedance portion 1 and the high impedance portion 2.
FIG. 22 shows an equivalent circuit that is approximated with a concentrated constant near the resonant frequency of the piezoelectric transformer shown in FIG. 21. In FIG. 22, Cd1 and Cd2 denote the constraint capacitance on the input side and the output side, respectively; A1 (input side) and A2 (output side) are force factors; m is an equivalent mass; C is an equivalent compliance; and Rm is an equivalent mechanical resistance. In this piezoelectric transformer, the force factor A1 is larger than A2, and in the equivalent circuit shown in FIG. 21, a voltage is stepped up by the two equivalent ideal transformers. Furthermore, since a series resonating portion constituted by the equivalent mass m and the equivalent compliance C is included, the output voltage becomes larger than the transformation ratio of the transformers, especially when a load resistance is large.
For the backlight of a liquid crystal display, in general, a cold cathode fluorescent tube having a cold cathode structure in which a heater is not provided in an electrode for discharge, is used. Since the cold cathode fluorescent tube has the cold cathode structure, the discharge start voltage at which discharge is started and the discharge maintaining voltage at which discharge is maintained are both very high. In a cold cathode fluorescent tube used for a liquid crystal display on the order of 14 inches, in general, 800 Vrms is necessary as the discharge maintaining voltage, and about 1300 Vrms is necessary as the discharge start voltage.
FIG. 23 is a block diagram of a separately-excited oscillation system driving circuit of a conventional piezoelectric transformer. In FIG. 23, reference numeral 13 denotes a variable oscillation circuit for generating an alternating driving signal for driving a piezoelectric transformer 10. An output signal from the variable oscillation circuit 13, which generally has a pulse waveform, is converted to an alternating current signal that is near a sine wave with its high frequency component being removed by a waveform shaping circuit 11. An output signal from the waveform shaping circuit 11 is voltage-amplified to a sufficient level to drive the piezoelectric transformer 10 by a driving circuit 12. The amplified voltage is input to a primary electrode 3U. The voltage input to the primary electrode 3U is stepped up due to the piezoelectric effect of the piezoelectric transformer 10 and is output from a secondary electrode 4.
A high voltage output from the secondary electrode 4 is applied to a series circuit of a cold cathode fluorescent tube 17 and a feedback resistor 18, and an overvoltage protection circuit 20. The overvoltage protection circuit 20 includes voltage dividing resistors 19a and 19b, and a comparing circuit 15 for comparing a voltage generated across the voltage dividing resistor 19a with a first reference voltage Vref1, and controls the variable oscillation circuit 13 via an oscillation control circuit 14 such that the high voltage output from the secondary electrode 4 of the piezoelectric transformer 10 is prevented from becoming higher than the preset voltage determined by the first reference voltage Vref1. The overvoltage protection circuit 20 is not operated while the cold cathode fluorescent tube 17 is turned on.
A feedback voltage generated across the feedback resistor 18 by current flowing the series circuit of the cold cathode fluorescent tube 17 and the feedback resistor 18 is applied to a comparing circuit 16. The comparing circuit 16 compares the feedback voltage with a second reference voltage Vref2 and outputs a signal to the oscillation control circuit 14 so that current flows substantially constantly through the cold cathode fluorescent tube 17. The oscillation control circuit 14 outputs a signal to the variable oscillation circuit 13 so that oscillation occurs at a frequency in accordance with the output signal from the comparing circuit 16. The comparing circuit 16 is not operated before the cold-cathode fluorescent tube 17 is turned on.
Thus, the cold-cathode fluorescent tube 17 is turned on stably. In the case of driving by a separately-excited oscillation system, even if the resonant frequency is changed by the temperature, the driving frequency follows the resonant frequency automatically.
The current flowing through the cold cathode fluorescent tube 17 is controlled so as to be constant by configuring a piezoelectric inverter in this manner.
In recent years, with high brightness of liquid crystal monitors and liquid crystal televisions, brightness required for a liquid crystal backlight is increased. In order to satisfy this demand, not one, but a plurality of cold cathode fluorescent tubes are used.
However, since the light emission control device outputs an input dc voltage as a high-voltage ac voltage, utilizing the resonance operation of the piezoelectric transformer, the following problem is caused in the case where the cold cathode fluorescent tube is connected in the manner as shown in FIG. 23. When one cold cathode fluorescent tube is turned on, the inverter output voltage is reduced, and therefore other cold cathode fluorescent tubes cannot be turned on.
In order to solve this problem, it is necessary to drive a plurality of piezoelectric transformers. However, in the conventional light emission control device shown in FIG. 23, a plurality of piezoelectric inverter circuits have to be provided in order to turn a plurality of cold cathode fluorescent tubes on simultaneously, which results in a complicated and large-scale circuitry.
For the purpose of solving this problem, JP 5-251784A discloses a thickness longitudinal vibration piezoelectric ceramic transformer for driving a plurality of loads, using a piezoelectric inverter circuit employing a piezoelectric transformer, and a method for producing the same. This publication describes that according to this thickness longitudinal vibration piezoelectric ceramic transformer and the method for producing the same, compactness, high efficiency, and multi-input and multi-output can be achieved.
For the purpose of solving the above-described problem, JP 8-45679A discloses a lighting device for a cold cathode fluorescent tube for driving a plurality of loads, using a piezoelectric inverter circuit employing a piezoelectric transformer. This publication describes that according to this lighting device for a cold cathode fluorescent tube, a lighting device for a cold cathode fluorescent tube that can turn on a plurality of cold cathode fluorescent tubes by a high voltage with a high frequency from one piezoelectric transformer can be provided.
According to this thickness longitudinal vibration piezoelectric ceramic transformer and the method for producing the same disclosed in JP 5-251784A, it is true that a plurality of loads can be driven by using this piezoelectric transformer. However, different voltages are applied to the plurality of loads each other, because of the relationship between the output impedance of the piezoelectric transformer and the load impedance. Therefore, it is impossible to control a plurality of loads independently by the driving control of the piezoelectric transformer, only with one piezoelectric inverter circuit.
Also with the lighting device for a cold cathode fluorescent tube disclosed in JP 8-45679A, it is possible to drive a plurality of cold cathode fluorescent tubes simultaneously by the piezoelectric transformer. However, in this driving method, the output voltage from the piezoelectric transformer becomes high, and when considering the space distance and the creeping distance with respect to the high voltage, it is unlikely that compactness of the device can be achieved. In addition, in the safety design, it is not preferable that a voltage of several thousands of volts is output constantly in the inside of the apparatus. Furthermore, since the cold cathode fluorescent tubes are connected in series, so that with only one piezoelectric inverter circuit, a plurality of cold cathode fluorescent tubes cannot be controlled independently.